The following U.S. Patents and Publications are incorporated herein by reference.
U.S. Pat. No. 4,668,199 discloses an exhaust system for an outboard motor that includes a main exhaust passageway extending through a partially water filled chamber in the drive shaft housing. An inlet idle relief passage connects the top of the chamber with the main exhaust passageway and an outlet passage connects the top of the chamber with the atmosphere.
U.S. Pat. No. 4,781,021 discloses a marine engine exhaust muffler assembly that includes inlet and outlet members, the axes of which are offset from each other. A muffler housing is disposed between the inlet and outlet members and forms a chamber having disposed therein an annular multi-layered cartridge. The layers have a multiplicity of perforations therein for passage of exhaust gases and spent engine cooling water there through. The water assists in keeping the muffler clean. An imperforate arcuate blocking plate is nested within the lower portion of the cartridge to permit accumulation of the spent engine cooling water within the cartridge to aid in noise reduction at certain engine speeds. The blocking plate is provided with a relief opening means for assisting in water drainage upon engine shutoff.
U.S. Pat. No. 4,952,182 discloses an exhaust relief system for an outboard motor that includes an exhaust chamber into which exhaust is discharged from the engine. A first passage in communication with the exhaust chamber provides contraction of the exhaust as the exhaust passes rearwardly, from which the exhaust is discharged into an expansion chamber which substantially surrounds the exhaust chamber. From the expansion chamber, the exhaust is routed through and contracted into a second passage in communication with the expansion chamber, after which it is discharged to atmosphere. The tortuous path provided by the exhaust relief system, along with the repeated expansion and contraction of the exhaust as it flows to atmosphere, provides a muffling effect at idle operation.
U.S. Pat. No. 7,001,231 discloses a water cooling system for an outboard motor that provides a water conduit that extends through both an idle exhaust relief passage and a primary exhaust passage. Water within the water conduit flows through first and second openings to distribute sprays or streams of water into first and second exhaust conduits which can be the primary and idle exhaust relief passages of an outboard motor.
U.S. Patent Application Publication No. 2013/0280970 discloses a marine propulsion system for propelling a marine vessel in water. The system comprises a marine vessel and an outboard motor that is coupled to the marine vessel. The outboard motor has a first inlet that receives intake air for combustion. A second inlet is located on the hull of the marine vessel and a conduit conveys the intake air from the second inlet on the marine vessel to the first inlet on the outboard motor. The system comprises an outboard motor that is coupled to a marine vessel, and that comprises an exhaust gas relief outlet that is located above the water when the outboard motor is at idle speed. A conduit conveys exhaust gas from the exhaust gas relief outlet to a discharge outlet located on the marine vessel.